An automatic transmission mounted in a vehicle such as an automobile vehicle generally has a transmission mechanism including planetary gear sets and frictional engagement elements such as clutches and brakes. These frictional engagement elements are selectively engaged to change power transmission paths passing through the planetary gear sets, thereby providing predetermined gear stages in accordance with the driving mode of the vehicle.
In recent years, there have been attempts to increase the number of stages of an automatic transmission to improve, for example, the fuel efficiency of a vehicle. This increases the number of planetary gear sets forming a transmission mechanism. With an increase in the number of planetary gear sets, the axial size of the automatic transmission increases to degrade the mountability of the automatic transmission in the vehicle. In particular, in a transverse automatic transmission whose axial direction agrees with the width direction of the vehicle, interference between the automatic transmission and a frame member, for example, extending in the longitudinal direction of the vehicle may be problematic. This makes the problem of the mountability of the automatic transmission significant.
To address the problem, for example, Patent Document 1 discloses an automatic transmission including a two-stage planetary gear set in which two of planetary gear sets forming a transmission mechanism are stacked one on the other and arranged radially on the inner and outer peripheries. This structure reduces the axial size of the automatic transmission as compared to the arrangement of the two planetary gear sets along the axis of the automatic transmission.
As shown in FIG. 4, in an automatic transmission 101 of Patent Document 1, a second planetary gear set 120 is located on the outer periphery of a first planetary gear set 110 so that the first and second planetary gear sets 110 and 120 form a two-stage planetary gear set 140. A third planetary gear set 130 is adjacent to the two-stage planetary gear set 140 on one axial side of the automatic transmission 101 (on the left of FIG. 4). This arrangement reduces the axial size of the automatic transmission 101 as compared to the arrangement of the first and second planetary gear sets 110 and 120 along the axis of the automatic transmission 101.